Help needed with exceeding API call limit

Newbie here.  What does "API call limit has been exceeded" mean?  I have seen this several times now in the last two days when using Cloudingo and Cirrus Insight for Gmail but don't really understand what it means or what to do about it.  I am no longer able to send emails to anyone with a record in Salesforce - which means my work is on hold!  We have Enterprise Edition with 10 licenses, though probably no one but me is doing a whole lot in Salesforces at any given time, if that's relevant.  Thanks so much for your help - I feel really stuck!

@Solo Admins ​@* Release Readiness Trailblazers * ​@Admin to Admin Academy ​@WIT Group, Chicago, US