This is the 4594th most frequent German word.
soliden
Solid, reliable, or sturdy.
Here, 'solides' is used as an adjective ('solid') to describe the stability and robustness of the 'Fundament' (foundation).
Er hat ein solides Fundament für seine Karriere gelegt.
He has laid a solid foundation for his career.
In this example, 'solide' describes the reliability or convincing nature of the 'Argumente' (arguments).
Wir benötigen solide Argumente für unsere Diskussion.
We need solid arguments for our discussion.
Here, 'solide' qualifies the financial condition of 'Das Unternehmen' (the company), indicating that it is stable or well-founded.