Oceans of ink, real and digital, have been spilled on this topic. It is simple to understand with an intuitive description, though, if we just ignore the pedantic urge to ruin it for everyone.
Formally:
Entropy is the measure of disorder in a system.
In more practical terms, this is a measure of how spread out (and therefore useless or unavailable for work) the energy in a system is.
This definition assumes a very important principle: that the universe is perpetually becoming more disordered as time moves forward, and it might even be stated that the inexorable increase in entropy over time gives time its thermodynamic directionality. This becomes a geometric rather than energetic argument if you dig in too deeply, but that will start wars so we’ll leave it for now.
(The order of statements above can be nitpicked, but much like the energy vs geometry discussion isn’t important here.)
An interesting consequence of this is that entropy exerts a force as an emergent effect. For example, there is no fundamental reason that all the gas molecules in a balloon aren’t concentrated in one small region of the balloon’s volume, allowing the balloon to collapse. Instead, there are simply many many more likely arrangements of those molecules that are more evenly spread out when their various interactions are considered, and though this effect is highly random and unpredictable (and unrepeatable) in any detail, nonetheless this random disorder has the effect of very reliably keeping balloons inflated at birthday parties the world over.
In fact, the rubber that the balloon is made from is composed of flexy, long-chain polymers and they have a tendency to coil up because there are many more arrangements of those rubber polymers that are tightly scrunched up than extended in a line, and this gives the rubber a large part of its elastic resistance. This is also an emergent effect of entropy, the “entropic force”.