C6H8O7 + 3NaHCO3 <---> Na3C6H5O7 + 3H2CO3
In other words, a molecule of citric acid reacts with three molecules of sodium bicarbonate to form a molecule of sodium citrate and three molecules of carbonic acid. Although an acid is reacting with a base, this is not a neutralisation reaction. In a neutralisation reaction, the products are water and salt; lemon juice is a weaker acid and baking soda is an organic salt. If you were to measure the pH of a solution containing the products, it would be lower than 7 (acidic).
Carbon dioxide, a gas, is produced by the reaction. Any time a chemical reaction produces a gas, it will be heavily favoured due to the large increase in entropy associated with gas formation. When this happens, you will notice the solution bubbling and foaming rapidly.
This reaction is very exothermic, meaning it releases a sizable amount of energy as heat when it proceeds. Therefore, product formation (including the evolution of CO2 gas and water) occurs rapidly and the solution heats up noticeably. Fortunately, lemon juice is sufficiently dilute that there are not any real safety concerns associated with the reaction, although it will definitely make a mess if you add enough lemon juice.