Pick a presetVoice and Podcast even out speech. Music is gentler. Strong makes everything louder. Then move the sliders if you want.
What the compressor doesIt lowers everything above the Threshold by the Ratio, then Make-up gain raises the whole track. The result: the quiet parts are closer to the loud parts.
What the limiter doesIt looks a few milliseconds ahead and lowers the volume so the peaks never go above the ceiling. That is how you avoid clipping and get a louder file.
Check the resultThe waveform shows the original faintly and the result in pink. The line under it says how much the loudest parts were lowered.
Compression is very easy to overdo: too much makes the sound flat and tiring. Start with a preset and use the smallest change that does the job. To set the loudness for YouTube, Spotify or a podcast, finish with the LUFS Normalizer. Long files can take a few seconds to process.