This article explains what the "gap limit" is in MultiBit HD.
If you create lots of payment requests using the "Request" button on the Send/Request screen you sometimes see a message that you have reached the gap limit.
To ensure your wallet can be synchronised correctly on different devices there is a limit on how many unpaid payment requests you can have all in a row. If this limit did not exist all wallets would have to explore forever looking for an address that could have received payment through some other unknown wallet that shared the same wallet words.
To get round this gap limit you can simply pay a small amount of bitcoin to your most recent payment request address. This resets the gap limit and you can then create more payment request addresses until you hit the next gap limit and so on.
For a more technical description of the gap limit please read the BIP 44 specification.