Hash Time Locked Contracts are used in the Atomic Swap technology for example, making it possible to exchange one coin/token type for another. HTLC ensures that both parties finish the required operation within a specified period of time.
For example: two people agree to exchange 1BTC for 20ETH via Atomic Swap ... the HTLC contract verifies that one party has sent the 1BTC and the other one has sent the 20ETH, within a specified period of time. Both parties have to provide a cryptographic proof of payment. In case of one party not doing this, the swap will be canceled and fund will be returned to the person who already sent his coins.