Although this question is not simple, I would rather answer it than what infinity actually is :D Even though we know relatively a lot about the Universe, it is still very little to be able to answer this question unequivocally (of course there are many more questions).

There are currently two theories about the dimensions of the Universe, one of which predicts that the Universe is infinite and the other surprisingly predicts that the Universe is finite.

But it is necessary to realize that the term “finality” can be perceived in such a way that there is one place in the Universe where we could not continue moving straight ahead but turn back.

If we look at the Earth in this way, it is also infinite (like every sphere) — we can always go forward, even if we pass the place we were already on.

Because we know when the Universe started (14 billion years ago), we expect that the finite expansion time of the Universe could not have been such that it had an infinite size — if we are moving at an arbitrarily large speed (but not infinite) for a certain (arbitrarily large) time, we cannot travel an infinite distance.

Unfortunately, such a simple reasoning may not hold true in the case of an accelerating Universe, so it is also possible that the Universe is infinite.

To be able to say what the dimensions of the Universe are, we need to have a functional theory that describes the behaviour of the Universe.

This should be a theory connecting the theory of relativity and quantum physics, and we have not yet succeeded in connecting these two diametrically different theories (which work independently and are confirmed by many experiments).

Hopefully, we will do better in the coming years — to have an idea, we understand and know the composition of 5% of the Universe so far, we know almost nothing about the other 95%.

Thus, you will not be dissatisfied with this answer, because based on our knowledge, it is not possible to answer better.