This refers to the presence of motile sperm "stuck" together and may indicate antibodies in the semen. If the sperm are agglutinated there are fewer free sperm available to move up towards the egg.
We sometimes use something called the MAR (mixed agglutination reaction) test to measure the level of antibodies in the semen that attack the sperm. The treatment available depends on how high the levels of antibodies are, so it can sometimes be important to have this test to determine the most suitable approach for your partner. If between 50 - 80% of the sperm are coated in antibodies, IUI will usually be used as a first step in any assisted fertility approach, assuming no other factors are present which suggests that IVF might be more appropriate. ICSI would usually be recommended if more than 80% of sperm have antibodies on them