An IP address is the unique address your device is addressable world wide on the internet. There are two versions IPv4 and IPv6 since the world is running out of IPv4 addresses since there are more and more allocated all the time (think "Internet of things" putting everything these days on the internet, they all need a unique IP or in the case of a home network at least one unique IP to stand in for all of the devices behind your router and NAT (network address translation) allows mutliple devices to use the one IP your ISP gives you). Internet providers (Verizon wireless if it is your phone, your cable provider if it's a cable modem) assign you IP address dynamically to you and you have no control over that. So no one can "use a different IP" by choice unless the they are using a VPN to hit the public internet elsewhere or jumping around to different networks like wifi hotspots at mcdonalds, coffee shop, or even trying to use your neighbors network. Typically when you get an IP address on a cable modem via DHCP (dynamic host configuration protocol) you get the same one you've had in the past since they are "leased" out for a period before you have to renew (and typically still get the same one). On LTE networks for phones, your address changes much more often as you move around to different parts of the cellular provider's network, and if your address changes while uploading or downloading something it will get interrupted -- but most services like youtube can get going again and avoid interruption by having enough video buffered. Usually you won't notice just browsing forums and such, but watching a video stream for instance you will get at least a brief stream interruption if your IP changes -- so most of the time as your roam the LTE network your address will stay with you, but if you fall off the LTE network and come back or if the provider is using different IP blocks they own for different parts of the network, you can easily get a different address. But in the end your device at home or your phone will have a world-wide publicly routable IP address that is unique to you!
Trying not to go too deep here. I've worked on devices for years that carry IP traffic and enjoy mucking around with networks -- it's just what I do and it's a hobby too. Back to saws!!!
Oh -- one last interesting fact -- an IPv4 address is a 32 bit number usually written as 4 numbers 8 bits each separated by dots i.e. 24.35.57.182 Each of the 4 can be the range 0 to 255 (min and max for 8 bits in binary). There are 2^32 unique IPv4 addresses so 4,294,967,296 are avialable in the IPv4 address space.
I could go on and on in many different directions of detail but will spare you guys...