conn=150 -> connection number 150. An increasing counter for each specific connection.
op=210 -> operation number within the connection number. An increasing counter for each operation for a specific connection.
So your example tells you that connection 150 has had 210 operations (there will be more, at least an unbind).
There is one special connection: conn=-1 fd=XX, this tells you that a specific connection is closed. Which connection can be found by matching the fd. I.e:
The start of a connection looks like this in a log:
slapd: daemon: conn=7448 fd=43 connection from IP=127.0.0.1:40629 (IP=:: 389) accepted.
After the search/add/mod the connection is closed with the following log entry:
slapd: conn=-1 fd=43 closed
Hope this clears things up a bit.