Means Innovation. This is the addition/removal/ignorance of an Islamic law or teaching. Bida usually goes against what is written in authentic hadiths and the Quran or goes against what the Prophet (PBUH&HP) practiced and taught.

The Prophet (PBUH&HP) said: "If anyone introduces an innovation in the religion, he will be responsible for it. (Good or bad). If anyone introduces an innovation or gives shelter to a man who introduces an innovation (in the religion) he is cursed by Allah, by His angels and by all the people." [Sunnan Abu Dawud]

The Prophet (PBUH&HP) also said: "Whoever does an action which we (Allah and His Messenger) have not commanded, it will be rejected." [Muslim - Sahih]

There are many understandings about what is or isn't bida, so your best bet is to ask your scholar.