A widow / widower may remarry. A man may be sealed (married for time and all eternity) to multiple women. A women may only be sealed to one man. Civil marriages (till death do you part) has no barring.
This can play out in a variety of ways. One example from my own family is my grandmother died relatively young (in her ~60’s). She and my grandfather were/are sealed. Later, my grandfather then choose to remarry a widow, but just as a civil marriage (till death due us part) for company in their elder years. Both my grandfather and the widow refer to their original spouse as “my eternal companion”, intend to be buried with them, and raise again with them.
A man can simply be sealed twice (were are VERY uncertain how the logistics of this works out in the eternities). A woman can choose to have her old sealing canceled, such as example when there has been an abusive divorce.
A civil marriage is simply a civil legal marriage, “till death do we part”. They are presided by local legality. They are acknowledged by LDS, and two LDS people may choose to only be civilly married (such as my grandpa and the widow).
A sealing is a covenant marriage under God’s power, husband and wife bound for the eternities*. A couple must be civilly married in order to be sealed. Both participants (a male and a female) must be faithful LDS.
- This is pending their keeping of their covenants with God.