To understand this let's understand first what is Limit order. 

A limit order is an order to buy or sell a security at a specific price or better. A buy limit order can only be executed at the limit price or lower, and a sell limit order can only be executed at the limit price or higher. 

Example:

For Buy:

If Reliance is being traded at 2100.30. If trader wants to buy reliance at a lower price then he/she can set the limit order stating that the order should be executed at 2100.20. This helps trader to take the trade at their levels. So when reliance will come to 2100.20 then only the order will be executed. However if the market price falls below this limit price when the order is placed then it will get traded at market price which is better than limit price.

For sell:

If Reliance is being traded at 2100.30. If trader wants to sell Reliance at a higher price then he/she can set the limit order stating that the order should be executed at 2100.40. This helps trader to take the trade at their levels. So when reliance will come to 2100.40 then only the order will be executed. However if the market price rises above this limit price when the order is placed then it will get traded at market price which is better than limit price.

Why sometimes trades set at Limit order gets executed as market oders?

The answer to this is that whenever you place a Limit order you are putting an limit in terms of price at which either you can buy or you want to sell.  In cases of very active stocks the movement of price is very fast, and also due to market volatility the prices keep moving up or down very fast.  As such the exchange matching orders get executed at the most advantageous price for the client.  But in any case the Limit price will not be breached, in case of buyer his trade may get executed at either the limit price or lower and in case of seller his trade may get executed at either the limit price or higher.

​For more details, refer to the CubePlus User Guide by Tradejini​