The full name of TTL is "Time To live". In short, it refers to the keeping time of DNS record at DNS server. Below is an example to explain what is TTL's value.
Supposing there is a domain myhost.abc.com (Actually this is a DNS record, it generally means that there is a host computer called myhost in the domain abc.com). its corresponding IP address is 1.1.1.1, and its TTL is 10 minutes. This domain or this record is kept in a DNS server called dns.abc.com.
Now if an user inputs the following address (or URL):
http://myhost.abc.com, what will happen next?
The DNS server 8.8.8.8 appointed by the user (or the DNS assigned automatically by his ISP, network provider) will try to explain him myhost.abc.com, surely the DNS server 8.8.8.8 can't resolve right away since it doesn't include the records of myhost.abc.com. But through the recursive search of DNS in the world, it finally goes to the server dns.abc.com.
The DNS server dns.abc.com informs the DNS server 8.8.8.8 the corresponding IP address of myhost.abc.com, and then 8.8.8.8 informs the user of the result. To speed up the resolving of this record, 8.8.8.8 will keep the result 1.1.1.1 for a period of time, that is called TTL time. In this period, if the user requests to resolve myhost.abc.com again, 8.8.8.8 will tell the user directly. While if the TTL time is outdated, the whole process will be repeated.