IMAP is a mail setting that may allow synchronization with phone and computer. In this case, the email is stored on the server.
The advantage of this is that if your laptop or computer hard drive crashes, your emails are safe.
The disadvantage is that IMAP can increase considerably in terms of storage space when used intensively and if you keep all emails. You will often need a larger hosting package in the long term or an email solution such as Office 365.
The counterpart of imap is POP3.