Tendons

TL;DR

something.

Tendons attach muscle to bone (and other structures such as the eyeball) and operate to move what they are attached to. Ligaments attach bone to bone (consider what holds together your fingers and toes together) and operate to hold structures together and keep them stable.

Tendons (and ligaments) develop and adapt far slower than muscle so require far more rest and time to recover than muscles.

Caution

Be careful when starting any new physically demanding hobby, sport or exercise routine. Your muscles recover relatively quickly, your tendons and ligaments do not. Don't go hard at the start - you're just making everything harder. If you've decided you'll enjoy running but you barely move all day (I'm trying to avoid using the phrase 'couch potato' here) I'd suggest you start by walking for a few minutes each day for a few weeks before you even think about running one kilometre or mile. If you're a sedentary, desk-bound office worker looking to lift weights, once a week and nothing heavier than the bar is the perfect place to start.

You may feel you haven't 'given it your all', it was too easy or you're ready to go again but if it's early in your fitness journey, or it hasn't even begun yet, take it slow and take it easy.