A. From a non-sexual perspective, we swallow on daily basis food, drinks and our own saliva. When in need we also swallow pills or other forms of medicine. Accidentally different objects may end up being swallowed as well (particular high risk for small children).
B. From a sexual perspective, it has two meanings:
1. Swallowing male/female cum (in general), however mainly referring to sperm. Sperm swallowing is a touchy issue, especially in hetero couples. Men see it as a proof of love - and can be hurt or unsatisfied if she spits it out. Most women however have a hard time adjusting to the taste (often quite bitter). But that's maybe one of the very reasons why men appreciate women who swallow: "Her love for me and/or craving for my seed is stronger than whatever taste it might have".
2. Allowing the head of the penis penetrate the throat during oral sex. Also known as deep-throating, the act requires lots of practice before it can be achieved. While cum swallowing is considered an act of love, deep-throating is mainly regarded as sexual performance.
B.1. "- What makes her so special?"
"- Not only does she swallow, but she swallows without making a face or anything. She even moans and looks up at me with happiness & love in her eyes. It melts my heart and gives me the greatest orgasms. I feel I'm the luckiest guy on Earth."
B.2. "Wow! I knew she was good, but when I saw the whole thing disappearing in her mouth, with like half of my 9" shaft swallowed down her throat, I was in total awe and divine happiness in the same time."