01In plain English
Plugins add sounds and effects that the host software does not have: a synth, a reverb or a noise remover. VST is the most common format; Apple's AU format does the same job on Mac.
02Why it matters when you are choosing
Plugins are how producers and editors build their sound. An app that cannot load the formats you already own means buying again or doing without.
03What to check
Ask the vendor
Check which formats the app supports (VST3, AU, AAX) and whether your existing plugins run on your operating system.