Citation managers are programs that help you cite your sources and organize those citations. Often, citation managers will also have some features to help you organize the information you gather from your sources.
If you struggle with keeping your sources straight or remembering which information came from which source, you may want to use a citation manager. Citation managers aren't perfect, so you will still want to look over any citations they help you write for errors before submitting your papers.
Not sure which citation manager to choose? Grafton Librarians suggest using Zotero. Mendeley is very similar, but Zotero has slightly better functionality in a few areas and is open-source. Both have free options with reasonable restrictions on file storage.