What is assembler directive?

can anyone help me in explaining what is assembler directive and what are software development tools

It means a command for the assembler to perform rather than an instruction for it to translate to machine code. For example, there could be a directive to store a literal value at the current address, or to control what information gets produced on an output listing, or to include another source file.

Edit: oops I forgot to explain that software development tools are programs that computer programmers use to help them create software, such as:
- compilers that translate human readable programs into machine language,
- debuggers that help you work out what a program is doing as it is executing,
- profilers that allow you to measure how much time each part of your program is spending,
- memory checkers that make sure you're not writing outside of arrays and allocated blocks.

Software Development Tool =
A programming tool is a program or application that software developers use to create, debug, or maintain other programs and applications. The term usually refers to relatively simple programs that can be combined together to accomplish a task, much as one might use multiple hand tools to fix a physical object.

Assembler Directive -
An assembler directive is a message to the assembler that tells the assembler something it needs to know in order to carry out the assembly process; for example, an assemble directive tells the assembler where a program is to be located in memory.

When you write a program in a high level language like C++ or Visual Basic it needs to be Assembled into a language the PC can understand. Machine Language.

Assembler directives are all the things in an assembler program that aren't actual executable code - things like defining a macro, or commands to the assembler on how to do things ("assume 16-bit math", "use 64 bit pointers", etc). The exact details will depend on the actual assembler in use - consult the program's documentation.

Software development tools - a collective term for all the software that a programmer r uses to get from a half-done idea sketched on a piece of paper to a fully finished program - they'll include editors, compilers, debuggers, and often other stuff like code analysis software, and source code management systems - when you're working on a project with 6 or 10 million lines of code, you need help just remembering which function is in which source file, and what programmer(s) are responsible for changes to it.