SMS is Short Messaging Service. This service is the oldest means of sending text based communications to another cellular user. SMS is the only supported option for basic phones that run either a proprietary OS, or running KaiOS. I personally feel that KaiOS should support RCS to expand the ability of RCS support for other users, but that is likely to not happen. As soon as the basic phone manufacturers either somehow support RCS, or they are no longer sold, then SMS can retire. Until then, SMS will continue to be a viable options for those that don’t have access to dedicated messaging apps.
Keep in mind, that SMS uses the voice channels of a cellular network to send the messages. Your carrier might offer you unlimited domestic SMS, but if you are planning on sending SMS messages to someone outside of your country, you would want to contact your carrier about costs of doing this. This is why iMessages, and RCS is so popular as they use data to send international messages. Also, even if your carrier will give you SMS at an affordable price, the recipient may not be able to reply to you because prices are unaffordable to them. If you are still using a basic phone, then you may want to consider an Android, or iOS device in your next upgrade as they will support RCS which is the planned successor to SMS.