Something that is porous has many small holes, so liquid or air can pass through, especially slowly.

porous soil with good drainage 有良好排水系统的渗漏性土壤

porous brick walls 多孔砖墙

a porous polymer membrane 多孔聚合物膜

not protected enough to stop people going through

The border in this region is porous and many refugees have simply walked across. 这个地区的边界管理很松懈，许多难民很容易就越过了。