Literally it's a 'cat's table', a small low table at which in former years well-heeled people fed their cats or small dogs. Such a table was sited in the dining room in a corner away from the dining table.
Today this German expression is used for a special table for smaller children at which they have to sit at family parties when there is not enough space at the grand table. Small children love it but when they grow up they often regard being sited there as a foreclosure.
'Katzentisch' is also used for that small, badly lit table in a restaurant's corner, where mainly solitary women were and still are placed. And it is used in politics and business to express that somebody (mostly male... ha-ha) has lost his influence. He is still there but without authority to act.