The firm rock is our Lord. Through Him we can walk confidently, since if we walk on this rock, we will never fall into the void.
For example, when we build a house on land that is not firm, what would happen? Well, the house will sink little by little, but when we build this house on strong and firm earth, it is very difficult for the house to sink or fall, because you knew how to build on good soil or on a rock that is totally firm.
That is why we make a brief comparison of who our Lord is for us, He is our sustenance and who cares for us day after day, so we must fully trust. God is our adviser our helper, only He is the one who guides our walk, who affirms our step is the Lord of hosts.