If you're circumcised, this will not concern you. For 99.9% of uncircumcised men and boys Phimosis is irrelevant. But for those of us who are unfortunate, then it is important. It's important enough to be taken very seriously. There are many resources available. This link is just one of them.
So, let's define it: Phimosis is a constriction of the tip of the foreskin which either prevents or seems to prevent it from retracting, either fully or completely.
Of course the medics' usual reaction is to say "Circumcision". And in a very few cases this may be the sole solution. But only in a few.
If the foreskin is simply tight, something which many, many boys and men have had over the ages, then it is a simple thing to deal with, as long as force isn't used. If we go all the way back to infancy, the only person who should ever touch a boy's foreskin is the boy himself. No-one else. Not the mother, not the father, and certainly no doctors! Much damage can be caused by some fool trying to withdraw a foreskin to "clean under it". If you want to argue the point, I'll tell you my medical history!
While doctors will classify and reclassify, there are really two types of phimosis. The "natural" one where the opening is not yet large enough, and that caused by damage and scar tissue from the damage. Each type is simple to resolve, though the second must be caught early, before more damage occurs.
For health, hygiene, proper sexual function, fun and much more, the foreskin must withdraw freely. But this is only truly necessary in adulthood. Around puberty it kind of becomes fun to withdraw the thing anyway! And when it withdraws easily, then it can be cleaned under. Gently and simply.
If simple withdrawal is impossible, visit your doctor, tell him that you do NOT wish to be circumcised, and ask for help and advice in gentle stretching and easing the opening. You should ask for "betnovate" ointment (a cortico-steroid with a thinning effect on skin) as an aid to stretching. This is a slow process. Rushing it will damage the skin and give you a serious problem: tears in the skin, and the formation of scar tissue.
Now scar tissue is a major challenge, because it isn't elastic. And you need it to be elastic to withdraw properly. Scare tissue means a visit to the doctor now, today. It can be cured if it's a small patch, with the same ointment and gentle stretching as a naturally tight foreskin. But if you damage it again it will increase the scar tissue and the cycle will go round and round again until it won't withdraw at all.
Even this is not a disaster. But it will take patient work with your doctor to avoid being circumcised.