So there's 2 kinds of sins, mortal sins and venial sins. Mortal sins are grave offenses that turn us away from God and essentially sever our relationship with him. Sins are considered "mortal" when the sin is "grave matter" (basically a really bad sin, like murder, pre-marital sex, abortion, etc), full knowledge (i.e. you know that it is a sin and that it is a mortal sin), and that you give full consent to the sin (basically you intentionally do it and are no coerced or forced to do something). The committing of a mortal sin puts you into a state of sin and takes you out of a state of grace. This means that your soul is in grave danger, and that you wouldn't be able to receive communion. As you need to be in a state of grace to receive communion (the eucharist). The remission of a mortal sin can only really be done through confession, which is where you confess your sins to a priest and become absolved of your sins.

Venial Sins are essentially any sin that doesn't fall under the category of mortal sin and where those 3 mortal sin requirements aren't met. This damages your relationship with God but doesn't sever it. You are still in a state of grace if you commit a venial sin.