The business model of the Gavagai API is pay-as-you-go. This means that you pay for what you use. But instead of paying for each and every call to the API, you choose a usage level that is appropriate for your needs and your budget, and that allows for a certain number of calls to the api each month. There are a number of levels available ranging from just a few calls per month to very have use with millions of calls per month. The full list of options is available on the API's pricing page. If your use exceeds the upper limit of your chosen level you will receive a warning message like the one in the title of this post: 'Usage limit exceeded'. You then have two opitons: either you stop using the API for the remainder of the current month and wait for a new month to start, or you upgrade your account to a higher level which allows for more calls. You can upgrade by logging in at developer.gavagai.se.
But I don't want to pay anything!
Not to worry; there is a free tier in the price list: it allows you to perform up to 100 api-calls with up to a total of 100 kB of data each month. This level you can use for as long as you want.