Most publishing contracts transfer an irrevocable license to the publisher to sell and distribute your book. There are different rights attached to ebooks, paperback, audio rights, and foreign rights. Some contracts only transfer the rights for some of these versions, other for all, so make sure you fully understand what you are exactly giving up. Especially audio book right has become a hot commodity, even if they are not still not as attractive to smaller presses, so this is something to look out for.
The other points to consider is the length of the license. Some publishers want perpetual, which means they carry on beyond your death (something I don’t recommend), others limit the license to a certain amount of years. Watch for automatic renewal clauses, automatic rights to sequels, prequels or spin-offs set in the same world, or other catch all phrases that could bind you to the contract beyond the agreed period.
Especially with smaller presses, there should always be a reversion clause. That means that certain events trigger the rights to be transferred back to you. Examples are if the publishers goes bankrupt, decides not to publish your book even after they committed, or when sales falls under a certain threshold.
In any event, I would always have a lawyer look over a contract before signing because there could be hidden clauses in there you might not understand but hurt you in the long run. Once you sign, you are locked in, and if something goes wrong, you face much higher legal bills which could have been avoided if you consulted a lawyer in the first place.