As mentioned,
Handbrake is one of my favorite go to free open source media conversion apps. It's default settings do a great job of reducing video file sizes while maintaining quality. Yet you have the ability to modify these settings if desired.
One handy feature is a free way to easily convert to and from HEVC (H.265) videos, popular with newer smartphones and cameras, to a more widely playable H.264 format.
Though my most common use is being able to convert variable frame-rate videos (i.e. iPhone) to constant frame-rate. Variable frame-rate is commonly used to reduce file size, though video editors can have difficulty in accurately playing these back, resulting in sync issues or dropped frames.
If your system has an nVidia graphics card, you can set this in Handbrake to potentially speed up conversions. The setting is located under the 'Video' tab, in the 'Video Encoder' drop-down options.