There is literally a "great chain of being" from high right down to low. Since God ordained it and it is thus a law of nature, any who oppose this order must be a little odd, if not outright dangerous. Still, as with the caveat at the start, there might be many ways to effectively step outside the restrictions of social status. However, you only can because people let you.
Despite life's profound unfairness, there is scope for social mobility, though not as much as you might think. Over generations, a family line might move from peasant, to wealthy peasant to peasant with tenants, to village leader, to knight, to manorial lord and then get lucky enough to get noticed and given some really big minor title by your overlord. In any one lifetime, you will not move much above your current station, those who had previously been above you will find this literally offensive and will oppose it with every fibre of their being. Though the church is often held up as a channel for advancement if you were blessed with intelligence and the right contacts, there is a suspiciously high (ie nigh universal) incidence of nobles in the ranks of the Saints, Church positions of abbot, bishop or above and even in those who get do intellectual, preaching and teaching work. Internally, the church as an organisation is heavily socially stratified with those who enter it being sorted into occupations based on their status. These occupations will, in addition to being more or less enjoyable and worthwhile of themselves, have different (or no) prospects for further advancement.
However, if you don't have social status of your own, you can always ride on someone else's coat-tails. They might let you because you are kin (see 'Kinship') or because you are useful in some way. Patronage is part of what holds society together, the powerful give benefits (offices, money, titles, gloves and so on) to those below them in return for service and so it continues down the chain reinforcing loyalties between lord and master. Patronage is not just socially beneficial, it is socially expected and if not performed, the offender will be punished in some way. However it works out in a given situation, any individual might come to enjoy quite a comfortable life full of opportunities and even a lot of power and freedom from the consequences of their actions, as long as their patron remains powerful. That's good for the patron, as their underlings know that sometimes literally their whole world depends on their Patrons success.
Here it is important to understand that this is a violent society. Not only are the nobles essentially just the biggest gangs in town and the key characteristic of males of the noble class is their possession of training and specialist equipment for warfare, those in charge do not claim any monopoly on violence. That's right, you are not going to hear anyone say "don't take the law into your own hands" here in the 15th Century. As long as your violence doesn't impact on their immediate interests, no one in power will be concerned if a group or individual chooses to mete out their own justice. Those on the top of the social order are there because they can fight to stay there, with so many gradations in the Medieval hierarchy, ultimately what is keeping "those beneath us" subordinate to their "betters" is force, and intimidation.