DNS Time To Live is a property of any DNS resource record. It defines how long this record can be cached by a client or any intermediary Domain Name System component (proxy, cache). Since information linked to record is not changing frequently and asking for a resolution is both time and resource consuming, it is important to be able to cache the queried information.
The TTL represents the number of seconds the record can be cached, thus considered to be still valid. Depending on the nature of the record and the usage, the TTL can vary from a few seconds (eg 20 for a CDN record like e6858.dsce9.akamaiedge.net, 60 for scontent.xx.fbcdn.net, 300 for www.google.com) or much longer (3600 for www.techcrunch.com).
Having a short TTL allows to be able to change the value of the record and shorten the propagation since the expiration in the cache will occur faster, but this generates globally more DNS traffic and impacts user experience if the authoritative server is distant from the user. Having a longer TTL limits the DNS traffic to authoritative servers and limits also the QPS (queries per second) it can handle.