When you write something, you own that written work and have the right to control who uses it and when. When you publish this work through a publisher, you are sometimes asked to transfer some of your rights to the publisher. The exact rights you transfer are listed in the agreement you sign. Often you are asked to transfer your copyright to a publisher, and this agreement is called copyright transfer.
Some copyright transfer agreements prohibit authors from distributing any copies of their final published article, some limit the distribution to pre-edited manuscripts, and some don't limit your ability to distribute at all. Make sure you are clear what your agreement was when you published your article. If you're not sure where to find that information, a good place to start is SHERPA/RoMEO, which can link you to specific journal/publisher copyright policies.
When publishing an article, you can ask the publisher to modify the terms of a copyright transfer agreement. Retaining the rights to distribute copies of articles for non-commercial uses is a frequent addition. You also may want to modify the agreement to provide for other rights, like the right to place a copy in PubMed Central. The Scholarly Publishing and Academic Resources Coalition (SPARC) has created an Author Addendum that you can use (or modify to fit your needs) to ensure you retain the rights you want.
The signing of the copyright transfer agreement is the only chance you have to negotiate for your rights to use your work. Consider what you’re signing before publishing your next article.