Grip is a consequence of molecular contact that can be measured to an incredibly small degree - about one hundredth of a micron - and is amplified when a car slides. Grip is created by molecular interaction at the point of contact between a tire and the track. When the tire is moving, part of the tread is physically touching the surface at a given point and its molecules extend until contact is broken.
Tire grip is generated by two mechanisms, sometimes referred to as physical grip and chemical grip. The first process involves the shear deformation of the contact patch, whilst the second involves the coefficient of friction of the tire. The internal stress response to shear deformation depends upon the shear modulus of the tire, which is temperature dependent, and the friction coefficient is dependent on both tire temperature and slip velocity. So both mechanisms by which grip is generated, are temperature dependent.
We can explain a grip as amount of traction a car can transfer on track trough contact between tires and road, and traction car has at any given point, thus affecting how easy it is for the driver to keep control through corners, during braking or accelerations. Grip depends of track condition, temperature of track or tires, and tire compound used as well as car global set-up. Available grip is finite value for certain part of the track.
It's well known that F1 drivers used to say that during Friday free practice, track condition (or grip) is not good, or that later during session track condition will "come to them". That mean that track condition (read grip) will improve as more rubber is laid on the track and dust, send and dirt is cleaned from tack by passing cars.
Same is valid when they say that they can't get rubbers work properly, they can't get proper temperature to the tires and so on. All that mean - "I don't have enough grip"
Grip also depend, as I sad before, how good is your global car setup, aerodynamical efficiency of your car, aerodynamical grip, mechanical balance, brake balance, car downforce, dynamic weight distribution, ride height, suspension setup and a combination of all that.
A lot to think about for a race engineer and driver.
More about tire and how the grip is created, read here.