what does lost mud circulation mean

Re todays announcement, "gas shows have been encountered [in a secondary objective] although analysis of these has been complicated by lost mud circulation".

I did a google search and get an explaination along the following:

"The loss of quantities of mud to a formation, usually in cavernous, fissured, or coarsely permeable beds, evidenced by the complete or partial failure of the mud to return to the surface as it is being circulated in the hole."

What does this mean in practice?? Is it a negative, or a standard obstacle to work around?

MJS