What do parentheses in a C variable declaration mean?

What are the parentheses for?

In C brackets [] have a higher precedence than the asterisk *

Good explanation from Wikipedia:

To declare a variable as being a
pointer to an array, we must make use
of parentheses. This is because in C
brackets ([]) have higher precedence
than the asterisk (*). So if we wish to declare a pointer to an array, we need to supply parentheses to override this:

double (*elephant)[20];

This declares that elephant is a
pointer, and the type it points at is
an array of 20 double values.

To declare a pointer to an array of
pointers, simply combine the
notations.

int *(*crocodile)[15];

Source.

And your actual case:

int (*data[2])[5];

data is an array of 2 elements. Each element contains a pointer to an array of 5 ints.

So you you could have in code using your ‘data’ type:

int (*data[2])[5];
int x1[5];
data[0] = &x1;
data[1] = &x1;

data[2] = &x1;//<--- out of bounds, crash data has no 3rd element
int y1[10];
data[0] = &y1;//<--- compiling error, each element of data must point to an int[5] not an int[10]