Sync_FF permit to synchronize a signal A in the clock domain clk. Normally, the A signal must be the output of a FF cloked in an other domain. You shouldtn't have "logic" between the 2 domain.
You can configure the FF number (default 2). This number is depending of the MTBF(Mean Time Between Failure).

p=. !{width: 25%}SYNC_FF.png!
COMPONENT SYNC_FF_LPP_JCP
GENERIC (
NB_FF_OF_SYNC : INTEGER);
PORT (
clk : IN STD_LOGIC;
rstn : IN STD_LOGIC;
A : IN STD_LOGIC;
A_sync : OUT STD_LOGIC);
END COMPONENT;

|.Parameter |.Type |.Size |.Description |.Default |
|NB_FF_OF_SYNC |Integer | |Number of FF |2 |
|\5.|
|.Signal |.Direction |.Size |.Function |. Active |
|clk |input |1 |clock |rising edge |
|rstn |input |1 |reset |low |
|A |input |1 |data in | |
|A_sync |ouput |1 |data out synchronized | |