It means the rated current that this contactor can switch for Load category AC1 (thermal loads),
Example: LC1D09 is 9 A on AC3 and its thermal current (Ith = 20 to 25 A) (this means it can switch up to 20 or 25 A for loads of AC1
Example: LC1D09 is 9 A on AC3 and its thermal current (Ith = 20 to 25 A) (this means it can switch up to 20 or 25 A for loads of AC1