Subtitles are captions that are usually displayed on the bottom side of the screen and are created to help viewers with hearing problems understand and follow dialogues. They also come in handy when watching films that contain dialogues in foreign languages as they can provide you with the translation of the conversations. Aegisub is a feature-rich program that helps you create and edit subtitle files on your Mac.
The application allows you to import various types of subtitle files (SRT, SSA, ASS, SUB, etc.), provides you with multiple text editing and timing options, and enables you to check if the resulted subtitle is in sync with the audio and video content of the movie. What's more, it comes with an integrated spell checker that helps you identify and modify spelling mistakes.
Once you finish editing your subtitle file you can preview it while watching your film, directly from Aegisub's interface. Just go to Video -> Open Video, select the movie, and the utility will display a small media player in its main window.
This program can be used to create movie subtitles, karaoke text content, and many others. The resulted files can be exported with their original format or converted to the ASS format (created by the Aegisub developers).
All in all, I think Aegisub is a reliable solution for creating and modifying subtitle files. It offers you many useful features, works with many subtitle formats, and is free.