In the lambda calculus, the single fundamental object type is functions. The Church encoding of the natural number $n$ is the composition operator: $$c_n = \lambda f. \lambda x. f (\ldots (f \, x) \ldots )$$ where $f$ is applied $n$ times.
The usual arithmetic operations can be encoded as operations on functions, for example: \begin{align} \mathrm{plus} &= \lambda m. \lambda n. \lambda f. \lambda x. m \, f \, (n \, f \, x) && \text{($$m+n$$ maps $$f$$ to $$m f$$ composed with $$n f$$)} \\ \mathrm{mult} &= \lambda m. \lambda n. m \, (n \, f) && \text{($$m \times n$$ composes $$(n f)$$ $$m$$ times)} \\ \end{align}