Indoors, a pop filter handles voice. Outdoors, even a small breeze ruins a recording. The fix is foam (light wind) or a fuzzy “deadcat” cover (real wind).
A windscreen is a foam cover that slides over the front of a microphone to block wind noise. The foam version is the slip-on bulb you see on the head of a handheld mic. The fuzzy “deadcat” (also called a windjammer or wind muff) is a furry cover that fits over a shotgun mic for outdoor use.
Indoors at a desk, you almost never need a windscreen — a pop filter handles plosives just fine. Outdoors is a different world. Even a light breeze hits a microphone diaphragm like a hurricane. Without a windscreen, your audio will be unusable.
Foam windscreens cut roughly 6dB of wind noise. Fuzzy deadcats cut 20dB or more. For serious outdoor work — film sets, sports broadcasts, news in the field — you need a deadcat, often combined with a Rycote-style basket windshield around the entire mic.