For resolving a domain name, it has to be registered and pointed to at least two name servers. These name servers will host the DNS zone file for that specific domain. When someone on the internet looks for the IP address of that specific domain name, the request will be first directed to the root name servers for the requested TLD (Top Level Domain like .org, .com, .net etc.). The root name servers will reply with the authoritative name servers and the request will then go to the server that is hosting the file along with the DNS records from where the IP will be resolved.
Every ISP (Internet Service Provider) has its own caching DNS server / servers. The DNS records are stored in these servers in their local cache. This helps in saving the network traffic from querying the authoritative name servers every time a request is received for the given domain. So when you enter your domain name in the browser and press enter, the request is directed to the name servers of your ISP. The IP address of your domain name is resolved from the authoritative name servers and the record is stored in the local memory of the ISP name server.
The stored record is maintained in the local name server memory in accordance with the settings applied in your DNS zone file and especially the value of the TTL parameter in the SOA record (Start Of Authority). By default this value is set to 14400 seconds (4 Hours) in the BIND name server. Frequently, this value is increased by a day or two due to traffic saving purposes.
When your local name server caches a record from your DNS zone file, the record is maintained in its memory. If you make another request for the same record, before the TTL time passes, the ISP name server will reply with the stored record instead of querying the authoritative name server again and this will keep repeating till the TTL time is re-cached. Once the TTL time is re-cached, your DNS zone will be fetched again from the authoritative name server for your domain and then the updated server records will be served.
This is the reason why you have to wait up to a day when you make any changes to the DNS zone.
While you wait for the updated record to get propagated, in the mean time you can flush the DNS resolver of your computer as the DNS records might be cached in your computer memory as well.