'Friction', also known as 'spaghetti', is a term used to describe inefficiency of communication between a group of people. This is an important concept to grasp in a complex MMO such as Eve Online, due to the sheer amount of possibilities and information available.
A small fraction of the information available is needed in the decision making process, so it is vital that communications remain as clear and concise as possible. If important commands such as primaries and align points have to be repeated over and over again to be heard, there is too much friction on comms.
Friction can turn the easiest of tasks into an exhausting mess and should be avoided when possible.
The following are a few examples of things you probably shouldn't ever say on a fleet. This is highly contextual and dependent on how many other people are talking, what the fleet is doing, etc.
Basically, before you talk on a fleet you should consider 'how will this information impact the fleet'. If the answer to this is 'not at all', you should consider keeping your mouth shut. There's a lot of exceptions so take this all with a pinch of salt.
Ironic FRICTION is still FRICTION.