The context useful to interpret it is the Aristotelian philosophy; much of A's metaphysical theories are a response to the ancient "riddle" of becoming (originated with the Eleatic school).
According to Aristotle's Metaphysics, intelligibility of "becoming" derive
from the concept of substance as a union of matter and form and from the principle that everything which becomes has an efficient cause which is the starting point for becoming.
Substance is therefore an act, the goal of becoming : the union of matter
and form into substance thus acquires a dynamic value. In this way, becoming is no longer a nullification of being, but is instead the union of what is possible (the power : the capacity to produce or undergo change) and what really is (the act : an object's existence).
The passage from potentiality (or power) to act always requires a cause. Something must make it happen. From this derives the axiom which governs the last two books of Aristotle's Physics :
If a thing is in motion, it is of necessity being kept in motion by something". (see also Aristotle on Causality and Aristotle's Natural Philosophy.)
Following Galileo, in Descartes' Physics (uniform) motion is nor more becoming; it is a "state", and thus it requires no cause.
The same holds for Newton's Physics :
Law I is the law of inertia : the state of rest, or of uniform motion in a straight line is preserved "unless it is compelled to change that state by forces".
What requires a cause (an "explanation") is the "alteration of motion", and Law II quantify the "capability" to generate motion of a force.
In conclusion, uniform motion, like rest, is not "becoming", and thus it does not require a cause.
This is the context for Descartes' leges naturae; see Principia philosophiae, Pars II, art.XXXVII :
Atque ex hac eadem immutabilitate Dei, regulae quaedam sive leges naturae cognosci possunt, quae sunt causae secondariae ac particulares diversorum motuum [...]. Harum prima est, unamquanque rem, quatenus est simplex et indivisa, manere, quantum in se est, in eodem semper statu, nec unquam mutari nisi a causis externis.
This is the first clear and complete statement of the law of inertia : "each thing, as far as is in its power, always remains in the same state; and that consequently, when it is once moved, it always continues to move". It is supported by law two [art.XXXIX] : "all movement is, of itself, along straight lines; and consequently, bodies which are moving in a circle always tend to move away from the center of the circle which they are describing."
Causa hujus regulae eadem est quae praecedentis, nempe immutabilitas et simplicitas operationis, per quam Deus motum in materia conservat.
Thus, conservation of movement is justified by God immutability, which means that the world acts according to laws established by God, that are simple and stable.
Nitpicking comment : the first law speaks of "unamquanque rem [...] semper in eodem statu perseveret", followed by an example regarding the shape of a body, while the second law speaks of "omnis motus". We can see in this wording an allusion to a more general principle of "immutability", form which it is deduced the more specific law of inertia.
[art.XL] Tertia lex naturae haec est: ubi corpus quod movetur alteri occurrit, si minorem habet vim ad pergendum secundum lineam rectam, quam hoc alterium ad ei resistendum, tunc deflectitur in aliam partem, et motum suum retinendo solam motus determinationem amittit; si vero habeat majorem, tunc alterum corpus secum movet, ac quantum ei dat de suo motu, tantundem perdit. [“a body, on coming in contact with a stronger one, loses none of its motion; but that, upon coming in contact with a weaker one, it loses as much as it transfers to that weaker body.”]
The (wrong) first rule of impact is justified, for the first part simply by the conservation of movement (by first law) and, for the second part :
[art.XLII] Demonstratur etiam pars altera ex immutabilitate operationis Dei, mundum eadem actione, qua olim creavit, continuo jam conservantis. [...] Deum ab initio, mundum creando, [...] ipsum conservando eadem action, ac cum iisdem legibus cum quibus creavit, motum, non iisdem materiae partibus semper infixum, sed ex unis in alias prout sibi mutuo occurrunt transeuntem, conservet. Sicque haec ipsa creaturarum continua mutatio immutabilitatis Dei est argumentum.
Thus, the world moves and changes without end, following the immutable rules [leges or regulae] established form the beginning by God : the overall "amount" of motion given ab initio to the world will not change, and this is the ultimate source of the behaviour of bodies in motion.