Christians from a Pentecostal background often talk about 'binding' negative/evil things. I am confused - do we as Christians have the authority to bind and loose spiritual things? My question is what is the binding and loosing that is spoken about in Matt 18:18? I really do not understand! Thank you.

I have always understood binding and loosing to be the apostles’ prerogative. The Roman Catholics, of course, believe that through apostolic success it applies to 'priests'. (There are no priests in the N.T., hence the inverted commas.) The context here is church discipline. It has nothing to do with binding demons, etc.

In Matthew 16 we also find binding and loosing, spoken of by Peter. What Peter proclaims “will have been bound in heaven” — in which case he is only playing a role in revealing God’s will.
In neither passage are modern interpretations of “binding Satan” and so forth appropriate. Satan (the “strong man”) is bound in Mark 3:27 and Luke 11:21, but the binding there is accomplished by Jesus, and probably refers to his death (see Col 2:13-15).