If your video and audio are recorded in separate files, you can synchronize them in the Media Panel. This process creates a new merged file, making it very easy to keep the sync locked while editing. Select the video file in the Media Panel, then hold CTRL (CMD on Mac) and select the associated audio file. Right-click on either of the files, and select "Merge Audio/Video" as shown in the image below.
HitFilm will compare the audio in both files, synchronize them, then combine the video frames with the separate audio file you selected. A new synchronized file will be created, and can then be edited on the timeline. The newly created file uses the same name as the original video, and appends a (Merged) tag to the name for differentiation, as shown in the below image.
This process does not create a new media file on your drive. It creates a referenced file, referencing the video stream of one file and the audio stream of the other, which requires very little storage space on your hard drive. If you wish to create a new standalone video file containing the synced audio and video, then you can add the Merged file to the timeline and export it.