Zero error arises when the measuring instrument does not start from exactly zero. Due to this zero error is present in each reading of the measure.

There are two types of zero error: Positive zero error and Negative zero error

Positive zero error:

If the zero marking of the Vernier scale lies on the right hand side of the zero marking of the main scale, then the error is said to be positive zero error. Look at the position of zero of the vernier scale on main scale.

Negative zero error:

If the zero marking of the Vernier scale lies on the left hand side of the zero marking of the main scale, then the error is said to be negative zero error. Look at the position of zero of the vernier scale on main scale

Least count of a measuring instrument is the least value of physical quantity that can be measured accurately using the instrument.

For example,

Take a spring balance whose range is 0-25kg. In this scale, you can see that the 1kg is divided into 10 equal parts. Therefore, the least value it can measure is 100g which can be found out by,

Measurement=1kg

Number of divisions=10

Least count=1/10 =0.1kg or 100g

Hence 100g is the least count of this instrument.

In the similar way you can find the least count of different spring balances of different range.