Recently i read many bonds names such as Elevated Railroad First 5s, United Drug Company 7s, etc. What does 5s or 7s mean?
It's just shorthand for the interest rate that the bonds pay. "5s" is short for "fives", which is short for "bonds paying a five percent coupon rate"; "7s" is short for "sevens", which is short for "bonds paying a seven percent coupon rate".
This terminology is still in use; when a company has more than one series of bonds outstanding, one way of distinguishing them is to refer to them as "5s" for the five percenters and "7s" for the seven percenters.
In some cases it is used for 'years' as in the paragraph below:
What immediately stands out on the chart above for me is that no matter what the Fed did, and by extension how the short and intermediate parts of the yield curve traded in relation to that, the long bond (for most of the time the 10s were it; the 30s disappeared in 2002 to come back later in 2006) by and large remained steady the whole time. There were, of course, variations, but for the most part the 10s ended up where they started.
Source: seekingalpha.com(emphasis mine)